1. Field of the Invention
The present invention relates to a device for judging which one of a plurality of mini discs is the currently played disc (i.e., the disc from which data read from a memory is derived) when these discs are played back normally or in succession.
2. Description of the Related Art
In recent years, mini discs (64 mm in diameter) which are smaller than the compact disc (CD) but permit writing and reading of substantially the same amount of data as the CD by the use of an information compression technology known as adaptive transform acoustic coding (ATRAC) have began to come into wide use. The MD (mini disc) uses a shockproof DRAM (dynamic random access memory) to prevent skipping of the reproduced sound due to vibration.
For example, compressed audio data recorded onto the disc is read at a data transfer rate of 1.4 Mbits/s by an optical pickup and written into the DRAM. The data is then read from the DRAM a data transfer rate of 0.3 Mbits/s and decoded by a decoder, expanded, and sent out. In this case, it is assumed that the DRAM has a capacity of 1 Mbits. The compressed audio data is read in in about 0.9 second and decoded in about 3 seconds while being read from the DRAM. If a large vibration is applied from outside at this time, and if the digital information on the disc cannot be read, reading of the compressed audio data from the DRAM is enabled for about 3 seconds. Therefore, if the optical pickup can go back to its original position on the disc within 3 seconds and restart the reading of the compressed audio data, then sound skipping can be prevented. For example, if the DRAM has a capacity of 4 Mbits, it follows that information corresponding to about 12 seconds is stored in the DRAM.
Because of the use of the DRAM described above, a slight time gap is present between the instant when data is read from the disc and the instant when the data actually arrives as sound to the user.
FIG. 7 is a detail block diagram of a mini disc (MD) player. Compressed audio data is recorded on an MD 20, which has a magnetooptical disc 21 rotatably held in cartridge 22. This cartridge 22 has a shutter 23. When MD 20 is placed in the record/playback position inside an MD player 100, a shutter-driving mechanism (not shown) opens the shutter 23 to permit a light beam from an optical pickup 4 to impinge on the recording surface of the magnetooptical disc 21. This also enables application of a magnetic field modulated by a recording signal fed from the magnetic head during playback. During reading, when the magnetooptical disc 21 is being rotated at a given rotational speed by a spindle motor 1, if the beam sharply focused by an objective lens 3 hits the recording surface of the magnetooptical disc 21, a reflected beam occurs which has a plane of polarization rotating according to the direction of magnetization of the recording signal. This reflected beam is read as an RF signal by the optical pickup 4 and sent to an RF amplifier 7, where the RF signal is amplified to a certain amplitude level. An address decoder 6 detects the frequency of wobbling (slight serpentine movement of the tracking grooves) of the RF signal. Even when no information is detected, positions in time on the magnetooptical disc 21 are detected.
On the other hand, during recording of data, the data is converted into digital form by an A/D converter 15 and then compressed to about one fifth of its original size by a data compression encoder/decoder 13. The signal is subsequently encoded into an EFM (eight-to-fourteen modulation) signal by an EFM encoder/decoder 9 and sent to a head driver circuit 5 and to a system controller 10.
The head driver circuit 5 drives a magnetic head 2 according to the entered EFM signal to magnetically modulate the magnetic film on the magnetooptical disc 21. The optical pickup 4 directs a light beam at the recording position on the magnetooptical disc 21. The beam heats the position above the Curie point, thus magnetizing this portion in the direction of the magnetic field given by the magnetic head 2. This direction of magnetization is a bit of information recorded.
The optical pickup 4 receives instructions from the system controller 10 to control the focus, tracking, and so forth via a servo controller 8. Also, the pickup 4 causes a carriage 14 to move the magnetooptical disc 21 into a desired radial position.
If alphanumerical keys 18 are operated to select a desired mode of operation or other information, the system controller 10 controls the operation of each component in response to the instructions and displays the mode of operation or the like on a display portion 17.
The data hierarchy of recorded data created on the MD is next described. FIG. 8 represents the data format of the MD. One cluster is composed of 36 sectors. One sector is 588.times.4 bytes=2,352 bytes. Compressed audio data is recorded in 32 sectors of these 36 sectors. The remaining sectors are used as link (discarded) sectors and sub-data recording sectors.
Two sectors are divided into 11, which are collectively known as a sound group. One sound group represents information about 512 samples (11.61 ms) of the left and right channels when the information is decoded.
The contents of one sector (2,352 bytes) are shown in FIG. 9. That is, one sector comprises a header of 8 bytes, an audio block of 2332 bytes, and a sync field of 12 bytes. Cluster/sector information is stored in the header. Compressed audio data is stored in the audio block. Information about synchronization is stored in the sync field. In this data format, the MD player reads data from the disc sector by sector and writes the data into the DRAM 12.
FIG. 10 shows one example of the allotment of data within DRAM 12. Main data, C2PO flag, and TOC (table of contents) data are separated and stored in their respective areas. Compressed audio data is stored in the main data area. Information about corrections to data is stored in the C2PO flag area. TOC data is stored in the TOC area.
For example, where the main data area in DRAM 12 has regions capable of accommodating 44 sectors of information, the main data (compressed audio) is written to and read from DRAM 12, using a ring buffer (44-nary counter) as shown in FIG. 11. When writing is started, data items contained in 1 sector are successively stored in DRAM 12 (e.g., data contained in 1 sector are in regions 00-01 shown in FIG. 11). Data contained in each sector are successively stored in regions 01-02, 02-03 up to regions 43-00, whereupon writing into DRAM 12 is inhibited. When reading is started, sectors stored in regions 00-01 are first read. Then, the sectors stored in the regions 01-02 and 02-03 are successively read. When the regions 00-01 become depleted, writing is restarted. In the state shown in FIG. 11, the next writing positions (the positions indicated by a write sector WR SCT that is a write sector number pointer regarding writing into DRAM 12 during playback) are locations 42 and 43. The next read positions (the positions indicated by a read sector RD.sub.13 SCT that is a sector read sector number pointer regarding reading from the DRAM during playback) are locations 01-02.
The TOC data is also stored in DRAM 12. This TOC data is stored at the header of the recording region of the magnetooptical disc 21 and contains information corresponding to the table of contents (TOC) of a book. Tune numbers, time information, and other kinds of displayed information are all controlled by the TOC data.
The general operation of the MD player constructed as described thus far, (e.g., two discs are played black consecutively). Will now be described by reference to the flowchart of FIG. 12. First, the TOC (table of contents) of one MD 20 held in MD player 100 is read (step 1201). Then, the information is played back from the MD 20 (step 1202). After completion of the playback, the currently played disc is changed to the next MD 20 (step 1203). Then, TOC is read from the next MD 20 (step 1201), followed by playback of the information (step 1202). A decision is made as to whether the disc has been changed (step 1203). If no disc change is made, given ending processing is performed, thus stopping the playback (step 1204).
In this way, the plural mini discs are exchanged successively and played back consecutively. If the currently played mini disc 20 is changed within the maximum information storage time of DRAM 12 (e.g., where the DRAM has a capacity of 4 Mbits, the time is about 12 seconds), information can be played back from the plural mini discs 20 without cessation.
When the first and second mini discs are played back continuously by the prior art MD player, information contained in the first MD and the information contained in the second MD may be stored together in the DPAM. In this case, it is impossible to judge whether the information read from the DRAM is derived from the first MD or from the second MD. Therefore, it is impossible to judge the origin of the information. Consequently, the MD player cannot inform the user as to whether the presently played disc has been changed. The instant of change may be displayed by detecting the timing at which the disc is changed by the disc changer. In this case, however, a slight time gap exists between the when the data is read from the disc and the instant when the sound actually arrives to the user as mentioned previously. Hence, the display presented to the user is not accurate.
If discs are to be discriminated, using the information read from the DRAM, it is impossible to gain access to the TOC data corresponding to the information read from the DRAM unless plural sets of TOC data are stored in the DRAM. Once again, the displayed data is not accurate.